13C NMR Study of Halogen Bonding of Haloarenes: Measurements of

Mar 13, 2004 - W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/journals/jacsat. Abstract: Solvent effects on t...
0 downloads 0 Views 334KB Size
Download PDF
Published on Web 03/13/2004

13

C NMR Study of Halogen Bonding of Haloarenes: Measurements of Solvent Effects and Theoretical Analysis‡ Rainer Glaser,*,† Naijun Chen,† Hong Wu,† Nathan Knotts,† and Martin Kaupp*,§ Contribution from the Department of Chemistry, UniVersity of Missouri, Columbia, Missouri, 65211, and Institut fu¨r Anorganische Chemie, UniVersita¨t Wu¨rzburg, Wu¨rzburg, Germany Received September 5, 2003; E-mail: [email protected]; [email protected]

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/journals/jacsat.

Abstract: Solvent effects on the NMR spectra of symmetrical (X ) F (1), X ) Cl (2), X ) Br (3), X ) I (4), X ) NO2 (5), X ) CN (6)) and unsymmetrical (X ) I, Y ) MeO (7), Y ) PhO (8)) para-disubstituted acetophenone azines X-C6H4-CMedN-NdCMe-C6H4-Y and of models X-C6H4-CMedN-Z (X ) I, Z ) H (9), Z ) NH2 (10)), 4-iodoacetophenone (11), and iodobenzene (12) were measured in CDCl3, DMSO, THF, pyridine, and benzene to address one intramolecular and one intermolecular issue. Solvent effects on the 13C NMR spectra are generally small, and this finding firmly establishes that the azine bridge indeed functions as a “conjugation stopper,” an important design concept in our polar materials research. Since intermolecular halogen bonding of haloarenes do occur in polar organic crystalline materials, the NMR solution data pose the question as to whether the absence of solvent shifts indicates the absence of strong halogen bonding in solution. This question was studied by the theoretical analysis of the DMSO complexes of iodoarenes 4, 9-12, and of iodoacetylene. DFT and MP2 computations show iodine bonding, and characteristic structural and electronic features are described. The nonrelativistic complexation shifts and the change in the spin-orbit induced heavy atom effect of iodine compensate each other, and iodine bonding thus has no apparent effect on Ci in the iodoarenes. For iodides, complexation by DMSO occurs and may or may not manifest itself in the NMR spectra. The absence of complexation shifts in the NMR spectra of halides does not exclude the occurrence of halogen bonding in solution.

Introduction

It has been known for a long time that the halogen atoms iodine, bromine, chlorine, and even fluorine can function as Lewis acids and engage in electron donor-acceptor interactions with atoms with lone pairs, such as nitrogen, oxygen, phosphorus, and sulfur.1 This interaction is now referred to as halogen bonding, as suggested by Legon,2 and the topic has recently been reviewed by Resnati.3 Halogen bonding may involve dihalogens X2 and X-Y as well as organic halides. Halogen bonding between organic halides and electron donors has been employed as an effective synthon in crystal engineering, and illustrative examples are provided by the work of Resnati4,5 and by Pennington and Hanks.6 Halogen bonding also has played a special role in studies of dipole alignment. Halogen ‡ Part 20 in the series “Stereochemistry and Stereoelectronics in Azines and Dienes”. For part 19, see ref 15a. † University of Missouri. § Universita ¨ t Wu¨rzburg.

(1) Bent, H. A. Chem. ReV. 1968, 68, 587. (2) (a) Legon, A. C. Angew. Chem., Int. Ed. 1999, 38, 2686. (b) Legon, A. C. Chem.-Eur. J. 1998, 10, 1890. (3) Metrangolo, P.; Resnati, G. Chem.-Eur. J. 2001, 7, 2511. (4) (a) Corradi, E.; Meille, S. V.; Messina, M. T.; Metrangolo, P.; Resnati, G. Angew. Chem., Int. Ed. 2000, 39, 1782. (b) Amico, V.; Meille, S. V.; Corradi, E.; Messina, M. T.; Resnati, G. J. Am. Chem. Soc. 1998, 120, 8261. (c) Valerio, G.; Raos, G.; Meille, S. V.; Metrangolo, P.; Resnati, G. J. Phys. Chem. A 2000, 104, 1617. 4412

9

J. AM. CHEM. SOC. 2004, 126, 4412-4419

Scheme 1. Atom Numbering of the Azinesa

a

The alkoxy groups in the unsymmetrical azines are in the Y-position.

bonding between iodine and nitro groups was claimed to yield parallel-aligned crystals of 4-iodo-4′-nitrobiphenyl,7 but the crystals structure was later shown to be disordered due to admixtures of 4,4′-dinitrobiphenyl.8 We accomplished nearperfect polar order in molecular organic crystals of unsymmetrical acetophenone azines (Scheme 1) using a rational design. The crystal structures of the (MeO,I)-azine,9 the (MeO,Br)azine,10,11 and the (MeO,Cl)-azine,12 respectively, are similar but differ in subtle details because of the occurrence of one, (5) (a) Cardillo, P.; Corradi, E.; Lunghim A.; Meille, S. V.; Messina, M. T.; Metrangolo, P.; Resnati, G. Tetrahedron 2000, 56, 5535. (b) Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G.; Stevenazzi, A. Cryst. Growth Des. 2003, 3, 799. (c) Navarrini, W.; Metrangolo, P.; Pilati, T.; Resnati, G. New J. Chem. 2000, 24, 777. (d) Burton, D. D.; Fontana, F.; Metrangolo, P.; Pilatim, T.; Resnati, G. Tetrahedron Lett. 2003, 44, 645. (e) Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G. Cryst. Growth Des. 2003, 3, 355. 10.1021/ja0383672 CCC: $27.50 © 2004 American Chemical Society

13

C NMR Study of Halogen Bonding of Haloarenes

ARTICLES

Figure 1. Crystal structure of (MeO,I)-azine 7 features perfect dipole-parallel alignment within 2-D layers and near-perfect dipole-parallel alignment in the direction in which the layers are stacked. The directionality of the halogen bonding might control the stacking of the layers.

Figure 2. Crystals of 4-iodoacetophenone azine 7 contain halogen-bonded strings. The halogen bonding is characterized by d(O‚‚‚I) ) 3.138 Å, ∠(CO‚‚‚I) ) 126.28°, and ∠(O‚‚‚I-C) ) 171.34°. The I-Ph bond approximately coincides with the direction of an O-sp3 lone pair.

two, and four independent molecules, respectively. The crystal structure of the (MeO,I)-azine (7) is shown in Figure 1. The perspectives of the halogen-bonded string shown in Figure 2 illustrate the O‚‚‚I-C geometry. A similar interaction of type (R2N)3PdO‚‚‚I-C was described by Zhu et al.,13 and the same group14 also recently described O‚‚‚Br-C and N‚‚‚Br-C interactions of the type observed in the (MeO,Br)-azine.11 This directionality of the halogen bonding (Figure 2) might control the stacking of the layers.15a We are studying halogen bonding for two reasons, and these address one intramolecular and one intermolecular issue. First, (6) (a) Crihfield, A.; Hartwell, J.; Phelps, D.; Walsh, R. B.; Harris, J. L.; Payne, J. F.; Pennington, W. T.; Hanks, T. W. Cryst. Growth Des. 2003, 3, 313. (b) Jay, J. I.; Padgett, C. W.; Walsh, R. D. B.; Hanks, T. W.; Pennington, W. T. Cryst. Growth Des. 2001, 1, 501. (c) Walsh, R. B.; Padgett, C. W.; Metrangolo, P.; Resnati, G.; Hanks, T. W.; Pennington, W. T. Cryst. Growth Des. 2001, 1, 165. (7) Jagarlapudi, A. R. P. S.; Allen, F. H.; Allen, F. H.; Hoy, V. J.; Howard, J. A. K.; Thaimattam, R.; Biradha, K.; Desiraju, G. R. Chem. Commun. 1997, 101. (8) (a) Masciocchi, N.; Bergamo, M.; Sironi, A. Chem. Commun. 1998, 1347. (b) Hulliger, J.; Langley, P. J. Chem. Commun. 1998, 2557. (9) Lewis, M.; Barnes, C.; Glaser, R. J. Chem. Crystallogr. 2000, 30, 489. (10) Glaser, R. E.; Chen, G. S. Dipole Aligned Molecular Materials with Nonlinear Optical Properties. U.S. Patent 6,229,047, May 8, 2001. (11) Chen, G. S.; Wilbur, J. K.; Barnes, C. L.; Glaser, R. J. Chem. Soc., Perkin Trans. 2 1995, 2311. (12) Lewis, M.; Barnes, C.; Glaser, R. Acta Crystallogr., Sect. C 2000, 56, 393. (13) Chu, Q.; Wang, Z.; Huang, Q.; Yan, C.; Zhu, S. J. Am. Chem. Soc. 2001, 123, 11069. (14) Zhu, S.; Xing, C.; Xu, W.; Jin, G.; Li, Z. Cryst. Growth Des. 2004, 4, 53.

we are examining one of our important design assumptions, namely, that the azine bridge functions as a “conjugation stopper”. We have previously studied acetophenone azines with theoretical methods16 and by 1H and 13C NMR spectroscopy in chloroform,17,18 and the results provide evidence that the azine bridge is indeed a conjugation stopper. Realizing that weak intermolecular interactions (e.g., halogen bonding) between solvent and solute may occur, corroboration by extension of the NMR measurements to a range of solvents is desirable. Second, we want to learn about the geometries and binding energies of halogen-bonded complexes of haloarenes because such interactions do occur in the polar organic crystalline materials we designed and prepared (Figures 1 and 2), and they may occur in solution. Solvent effects were measured on the NMR spectra of the symmetrical azines with X ) Y ) F (1), Cl (2), Br (3), I (4), NO2 (5), and CN (6), the unsymmetrical azines with X ) I and (15) Reviews: (a) Glaser, R.; Knotts, N.; Wu, H. Chemtracts: Org. Chem. 2003, 16, 443. (b) Lewis, M.; Wu, Z. Glaser, R. Arene-Arene Double T-Contacts. Lateral Synthons in the Engineering of Highly Anisotropic Organic Crystals. In Anisotropic Organic Materials-Approaches to Polar Order; Glaser, R., Kaszynski, P., Eds.; ACS Symposium Series, Volume 798; American Chemical Society: Washington, DC, 2001; Chapter 7. (16) Glaser, R.; Chen, G. S. J. Comput. Chem. 1998, 19, 1130. (17) (a) Lewis, M.; Glaser, R. J. Org. Chem. 2002, 67, 1441. (b) Lewis, M.; Glaser, R. J. Org. Chem. 2002, 67, 7168. (18) See also: Neuvonen, K.; Fu¨lo¨p, F.; Neuvonen, H.; Koch, A.; Kleinpeter, E.; Pihlaja, K. J. Org. Chem. 2003, 68, 2151. J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004 4413

ARTICLES

Glaser et al.

Y ) MeO (7, 4-iodoacetophenone-(4-methoxy-phenylethylidene)hydrazone) or Y ) PhO (8, 4-iodoacetophenone-(4-phenoxyphenylethylidene)hydrazone), two models (imine 9, Z ) H; hydrazone 10, Z ) NH2), as well as 4-iodoacetophenone 11 and iodobenzene 12. The theoretical analysis focuses on DMSO complexes of iodoarenes 4 and 9-12 for reasons given below. Experimental Section Iodobenzene, 4-iodoacetophenone, and hydrazine hydrate were purchased from the Aldrich Chemical Co. 4-Iodoacetophenone was recrystallized from ethanol. The preparation of 4 was described previously. 4-Iodoacetophenone hydrazone was prepared (yield >95%) by refluxing 4-iodoacetophenone and hydrazine hydrate (ratio 1:3) in ethanol (4 h). The deuterated NMR solvents CDCl3, DMSO, THF, pyridine, and benzene were employed. CDCl3 and deuterated DMSO were purchased from Cambridge Isotopes Laboratories, and deuterated THF, pyridine, and benzene were purchased from the Aldrich Chemical. The 13C NMR sample preparation of the (I, MeO)-azine 7 in DMSO required heating and sonication. The heteronuclear multiple quantum correlation (HMQC) method was used to help identify the peaks by establishing the C-H connectivities. The HMQC spectrum of the symmetrical (I,I)-azine 4 in DMSO shows cross-peaks at 128.4, 137.0, and 14.5 ppm, which correspond to Co, Cm, and methyl-C, respectively. The 13C NMR spectrum of 4 shows only five peaks. The APT NMR was recorded for 4 in DMSO, and it showed the expected three positive signals for the quaternary carbons Caz, Cp, Ci, and two negative signals for Co and Cm. Importantly, the two signals for Cp and Co, a positive signal for Cp and a negative signal for Co, are nearly isochronous with δ(Cp) ) 137.267 and δ(Co) ) 137.239 ppm. A DEPT spectrum provided further confirmation. The spectra are provided as Supporting Information. The 1H- and 13C NMR spectra were measured with a 300 MHz instrument (operating frequencies for 1H and 13C of 300.1317008 and 75.4769164 MHz, respectively) and a 250 MHz NMR spectrometer (operating frequencies for 1H and 13C of 250.1315321 and 62.9023694 MHz, respectively). The chemical shifts were assigned by comparison to empirical data.17

Theoretical and Computational Section Structures and Vibrations of Halogen-Bonded Complexes.19,20 Structure optimizations were performed at the MP2 level21 and with the B3LYP implementation of hybrid density functional theory. The effective-core-potential valence basis set LANL2DZ22 was employed for all optimizations (basis set A). Symmetric molecules were optimized with the appropriate symmetry constraints. Vibrational frequency analyses were carried out at the level of optimization to confirm that a stationary structure indeed had been located, to confirm the character of the stationary structure (minimum: all eigenvalues are real), and to (19) Essentials of Computational Chemistry, Cramer, C. J., Ed.; John Wiley & Sons: Chichester, U.K., 2002. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (21) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (22) (a) Dunning, T. H.; Hay, P. J. In Methods of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1977; Vol. 2. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284. (d) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. 4414 J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004

Figure 3. Computed MP2 structures of the iodine bonding complexes between DMSO and 4-iodoacetophenone imine 9, 4-iodoacetophenone hydrazone 10, 4-iodoacetophenone 11, and 4-iodobenzene 12.

W For the DMSO comples of 12, animated GIFs are avaliable of the six lowest vibrations with frequencies W 7 cm-1 (torsion mode of the ∠(SO‚‚‚Cipso-Cortho) dihedral angle Λ), W 20 cm-1 (rotational motion about the S-O bond), W 22 cm-1 (∠(S-O‚‚‚I) angle bending Ω), W 48 cm-1 (∠(O‚‚‚I-Cipso) angle bending Γ), W 52 cm-1 (∠(S-O‚‚‚I) angle bending in combination with ∠(O‚‚‚Cipso-Ph) bending), W 106 cm-1 (d(O‚‚‚I) stretching mode).

obtain thermochemical data. The potential energy surfaces of the complexes are rather flat with regard to the precise relative position of the iodoarene and the DMSO molecule, and most optimization of the complexes required optimizations with computed gradients and curvatures at all steps. Molekel 4.223 was employed to examine molecular vibrations and to generate the animations provided as web-enhanced objects. The vibrational analysis of 4‚(DMSO)2 could not be carried out at the MP2 level even with the modern equipment. Total energies and thermochemical data (Table S3) and the complexation energies (Table S4) are provided as Supporting Information. Molecular models of all optimized structures are shown in Figures S1-S4 in the Supporting Information, and the MP2 structures of the DMSO complexes are shown in Figures 3 and 4. Halogen Bonding Strength. Binding energies, enthalpies, and free Gibbs enthalpies were determined at the level of optimization. Total energies also were recomputed with those structures but employing an all-electron basis set. The 6-31+G* basis set was used for H, C, N, O, and S. For I, the 3-21G* basis set was used with augmentation by single diffuse s- and p-functions with the exponents 0.0468 and 0.0286 recommended by Radom et al.24 (basis set B). We also employed a basis set C which is like basis set B except that it replaces the 3-21G* basis set with the 6-311G* basis set for iodine.24 For iodobenzene, we applied coupled-cluster theory which considers all single and double excitations variationally and also includes triple excitations via a perturbational scheme, CCSD(T).25,26 CCSD(T) calculations employed basis sets B and C and were based on the MP2/A structures. (23) (a) Molekel, version 4.2; Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. Swiss Center for Scientific Computing: Manno, Switzerland, 20002002. (b) Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766. (24) Glukhovtsev, M. N.; Pross, A.; McGrath, M. P.; Radom, L. J. Chem. Phys. 1995, 103, 1878. (25) (a) Purvis, G. D.; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910. (b) Scuseria, G. E.; Schaefer, H. F. J. Chem. Phys. 1989, 90, 3700.

13

C NMR Study of Halogen Bonding of Haloarenes

ARTICLES

Results and Discussion

Solvent Effects on 13C NMR Spectra. The solvent contributes in several ways35,36 to the solvent screening constant σsolvent (eq 1). The term σa arises from the solvent magnetic anisotropy,

Figure 4. Computed MP2 structures of the doubly iodine bonded complexes between two molecules of DMSO and azine 4. The Ci and the C2 symmetric complexes are minima.

Nonrelativistic NMR Chemical Shift Calculations. The gaugeincluding atomic orbital (GIAO) method is a proven, efficient, and accurate method for the calculation of NMR shielding tensors,27 and GIAO NMR calculations were carried out at the B3LYP and MP2 levels with basis set B. Spin-Orbit Corrections to Shielding Constants. Spin-orbit (SO) effects are well-known to be particularly important for carbon atoms directly bonded to iodine.28 We therefore used the combined finiteperturbation/sum-over-states approach of ref 29 to compute SO corrections to the 13C shifts of carbon atoms directly bonded to iodine (Ci). The calculations were performed for the B3LYP/A- or MP2/A-optimized structures, respectively, using the gradient-corrected P86 functional,30 IGLO-II basis sets,31 and a finite-perturbation parameter λ ) 10-3. The common gauge origin for the magnetic vector potential was chosen to be on the nearest iodine nucleus. One- and two-electron SO matrix elements were computed within the atomic meanfield approximation,32 using the AMFI code.33 The finite-perturbation Kohn-Sham SCF calculations employed the deMon program,34 and the subsequent perturbation treatment employed the corresponding property module.29 (26) (a) Urban, M.; Noga, J.; Cole, S. J.; Bartlett, R. J. J. Chem. Phys. 1985, 83, 4041. (b) Noga, J.; Bartlett, R. J. J. Chem. Phys. 1987, 86, 7041. (c) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. (27) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc., 1990, 112, 8251. (28) Kaupp, M.; Malkina, O. L.; Malkin, V. G.; Pyykko¨, P. Chem.-Eur. J. 1998, 4, 118. (29) (a) Malkin, V. G.; Malkina, O. L.; Salahub, D. R. Chem. Phys. Lett. 1996, 261, 335. (b) Malkina, O. L.; Schimmelpfennig, B.; Kaupp, M.; Hess, B. A.; Chandra, P.; Wahlgren, U.; Malkin, V. G. Chem. Phys. Lett. 1998, 296, 93. (c) Vaara, J.; Malkina, O. L.; Stoll, H.; Malkin, V. G.; Kaupp, M. J. Chem. Phys. 2001, 114, 61. (30) (a) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (b) Perdew, J. P.; Wang, Y. Phys. ReV. B 1986, 33, 8800. (31) Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR-Basic Principles and Progress; Springer: Heidelberg, Germany, 1990; Vol. 23, 165 ff. (32) Hess, B. A.; Marian, C. M.; Wahlgren, U.; Gropen, O. Chem. Phys. Lett. 1996, 251, 365. (33) AMFI code: Schimmelpfennig, B. Atomic Spin-Orbit Mean-Field Integral Program; University of Stockholm: Stockholm, Sweden 1996. (34) (a) deMon program: Salahub, D. R.; Fournier, R.; Mlynarski, P.; Papai, I.; St-Amant, A.; Ushio, J. In Density Functional Methods in Chemistry; Labanowski, J., Andzelm, J., Eds.; Springer: New York 1991. (b) St-Amant, A.; Salahub, D. R. Chem. Phys. Lett. 1990, 169, 387.

σsolvent ) σa + σb + σw + σe + σs

(1.1)

σsolvent ) σns + σs

(1.2)

and this term is positive for disk-shaped solvents (e.g., benzene) and negative for rod-shaped solvents.35 The term σb relates to the bulk diamagnetic susceptibility of the solvents. The terms σw and σe account for van der Waals interactions and electrostatic interactions (“polar effects”), respectively, of the solvent. Finally, σs considers the effects of specific complexation. To assess the importance of specific halogen bonding, one needs to measure the spectrum in two solvents that either do not (e.g., CDCl3) or do (e.g., DMSO) allow for complexation. Since the combined effects of the first four terms (nonspecific shielding, σns ) σa + σb + σw + σe) might serendipitously offset any effect of halogen bonding, it is good practice to examine several solvents of each kind. Hence, we employed pyridine and THF in addition to DMSO as halogen bond donor solvents, and we also employed benzene as a second non-Lewis base solvent. Furthermore, our study of symmetrical azines includes some azines with nonhalogen substituents. If the effects of the nonspecific solvent shielding would serendipitously offset any effect of halogen bonding in the halogen-substituted azines (e.g., 1-4), then one should expect a significant observable nonspecific effect in the absence of halogen bonding opportunities (e.g., 5 and 6). The chemical shifts of 1-6 were measured in CDCl3 and DMSO, and the data show no abnormal changes (Table S1 in Supporting Information). Azine 4, most likely to engage in halogen bonding, was measured in other solvents as well, and no significant chemical shift changes were found (Table 1). The chemical shifts of Ci vary by no more than 1.2 ppm, and changes of this magnitude are “normal.” Similarly, no major solvent effects are observed in the NMR spectra of the unsymmetrical azines 7 and 8 (Table 1). Thus, our NMR experiments do not show any unusual effects. Since azines are conjugation stoppers and any halogen bonding would be local, the solvent effects on iodo-substituted azines should be very similar to the solvent effects of 4-iodoacetophenone hydrazone 10. We prepared this hydrazone, and its NMR spectra data are given in Table S2 in the Supporting Information along with the respective data for the related 4-iodoacetophenone 11 and iodobenzene 12. The chemical shift of Ci varies by less than 1.1 ppm as with the azines. However, a significant solvent effect of ∆δ ) -5.7 ppm does occur for Caz in that this atom’s chemical shift decreases from 146.0 ppm in CDCl3 to 140.7 ppm in DMSO. There is some specific interaction affecting Caz. The data in Table S2 show that the chemical shift of the amino-H changes from 5.4 ppm in CDCl3 to 6.5 ppm in DMSO due to hydrogen bonding between the amino group and solvent oxygen, which deshields the H-atom. The data resolve the intramolecular issue stated in the Introduction. Solvent effects on 13C NMR spectra are generally (35) Buckingham, A. D.; Schaefer, T.; Schneider, W. G. J. Chem. Phys. 1960, 32, 1227. (36) Foster, R. Organic Charge-Transfer Complexes; Academic Press: London, 1969; Chapter 5. J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004 4415

ARTICLES

Glaser et al.

Table 1. Experimental 13C Chemical Shifts (ppm) of 4-Iodoacetophenone Azine 4, 4-Iodoacetophenone-(4-methoxyphenyl-ethylidene)hydrazone 7, and 4-Iodoacetophenone-(4-phenoxyphenyl-ethylidene)hydrazone 8 as a Function of Solvent (I, I)-azine 4 solvent

Ci

Co

Cm

Cp

Caz

CH3

CDCl3 DMSO THF benzene pyridine

96.2 96.9 96.7 96.4 97.2

137.5 137.2 138.3 137.7 137.9

128.3 128.5 129.2 128.6 128.9

137.7 137.3 139.0 138.2 138.2

157.3 157.1 158.4 157.9 157.9

14.8 14.6 14.5 14.4 14.7

Ci

Co

Cm

Cp

Caz

CH3

CHCl3 DMSO THF benzene pyridine

95.9 97.0 96.3 96.0 97.0

137.5 137.3 138.2 137.6 137.8

128.3 128.2 128.9 128.8 128.9

138.1 137.7 139.3 138.6 138.6

157.3 157.2 158.2 157.8 157.9

14.9 14.5 14.6 14.7 14.8

Ci′

Co′

Cm′

Cp′

Caz′

C′H3

CHCl3 DMSO THF benzene pyridine

161.0 160.8 162.2 161.5 161.6

113.7 113.9 114.3 114.0 114.2

128.1 128.6 129.2 131.6 131.5

131.0 130.5 131.5 130.7 130.9

158.0 157.2 159.1 158.8 158.7

14.7 14.7 14.3 14.3 14.6

(MeO, I)-azine 7

(PhO, I)-azine 8

CDCl3 DMSO

CDCl3 DMSO-d6

Ci

Co

Cm

Cp

Caz

CH3

96.0 96.9

137.5 137.3

128.3 128.5

138.0 137.5

157.4 157.2

15.0 14.7

Ci′

Co′

Cm′

Cp′

Caz′

C′H3

159.0 158.5

119.4 119.4

129.9 128.5

133.2 132.9

157.7 157.3

14.7 14.5

small. Independent of the choice of the NMR solvent, hardly any electronic communication is found between the two halves of the unsymmetrical azines, and the spectroscopic properties of the symmetrical azines carry over to the unsymmetrical azines. But the NMR data pose a dilemma as far as the intermolecular issue is concerned, that is, the occurrence of halogen bonding. Gao and Goroff37 reported an unusual solvent effect in the 13C NMR spectra of diiodohexatriyne, I-CtCCtC-CtC-I, and of diiodooctatetrayne, I-CtC-CtC-Ct C-CtC-I, in CDCl3 and DMSO. The chemical shift of CR in C6I2 shifted from 0.9 ppm in CDCl3 to 14.6 ppm in DMSO. For C8I2, the chemical shift changed from 1.9 ppm in CDCl3 to 17.9 ppm in DMSO. These 13C downfield shifts were attributed to halogen bonding between iodine and DMSO. Hence, we must ask whether the absence of solVent shifts for haloarenes indicates the absence of strong halogen bonding in solution. Our theoretical analysis of DMSO complexes of iodoarenes will provide an interesting answer to this question. This system was selected for theoretical analysis because this halogen bonding is one of the stronger intermolecular interactions and because of its relevance to our materials (Figures 1 and 2). Of course, the dilemma exists only if complexation does occur and if complexation causes a chemical shift change. The complexation of iodoarenes by chloroform38 is very weak, and the weak complexation does not cause any complexation shift in the NMR spectra. We consider solute-solvent interactions (37) (a) Gao, K.; Goroff, N. S. J. Am. Chem. Soc. 2000, 122, 9320. (b) Rege, P. D.; Malkina, O. L.; Goroff, N. S. J. Am. Chem. Soc. 2002, 124, 370. 4416 J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004

only because this interaction is conceptually rather different from any solute-solute aggregation because of the entirely different entropy situations. The solute-solute aggregation may involve solutes that are different (e.g., ion pairing) or the same (selfassociation, ion pair aggregation). Any such aggregation requires significant solute-solute bonding to overcome the entropy term. Charge-charge interaction allows for ion pairing in solvents of low polarity.39 Ion pair aggregation is rare and requires less polar solvents.40 Association between solutes that interact only by dipole-dipole interactions is still less likely, and the selfassociation of water in chloroform is one of the very few examples.41 While all the solute-solute aggregates considered in that study allow for rather strong solute-solute intermolecular binding, only the water dimer was observed in chloroform. In particular, these considerations suggest that halogen bonding between two solute molecules does not occur. In crystals, iodine bonding to sp2 N is of course possible, and there are examples in the literature.14,42,43 For the azines, however, the structural record contains no examples of halogen bonding involving an azine N. We determined the crystal structures of the (F, F)-, (Cl, Cl)-, (Br, Br)-, and (I, I)-azines 1-4, and none features halogen bonding to an azine N,44 even in the absence of competition for halogen bond acceptors. Halogen Bonding between Iodoarenes and DMSO. IodoDonor Molecules. The optimized structures of 4 and 9-12 are shown in the Supporting Information. The structures of 9-12 are unremarkable, while 4 warrants a brief discussion. At the MP2 level, C2h-4 is a second-order saddle point structure (i31.8 and i21.7 cm-1), C2 and Ci minima were located, and they are almost isoenergetic (∆Erel ) 0.23 kcal/mol preference for the C2 structure, ∆Grel ) 0.4 kcal/mol preference for the Ci structure). At the B3LYP level, the C2 structure again is preferred but the C2h-4 now is a minimum, and an essentially isoenergetic Ci structure is a transition state structure (i1.9 cm-1, C2 distortion). While the details of the potential energy surface are subtle and therefore also sensitive to the theoretical model, it is clear that the azine can access a large conformational space at regular temperature. Halogen-Bonded DMSO Complexes of 9-12 and 4. The structures of the complexes between 9-12 and DMSO are shown in Figure 3, and all complexes are chiral. The structures of the doubly halogen-bonded DMSO complexes of 4 are shown in Figure 4, and the phenyl twist angles φ and the τ values remain unaffected by complexation. The van der Waals radii of O and I are 1.52 and 1.98 Å,45 and their sum is 3.5 Å. The O‚‚‚I distances in all complexes are close to 2.9 Å and significantly shorter (∼0.6 Å) than the sum of the vdW radii. Parthasarthy et al.46 mined the crystallographic record for evidence of directional preferences of intermolecular forces (38) Two complexes of iodobenzene and chloroform (both C1) were considered; one complex involves two hydrogen bonds and the other features one IH4C6-H/Cl contact (but no chlorine-iodine contact). (39) Dedonder-Lardeux, C.; Gregoire, G.; Jouvet, C.; Martrenchard, S.; Solgadi, D. Chem. ReV. 2000, 100, 4023. (40) (a) Gareyev, R.; Ciula, J. C.; Streitwieser, A. J. Org. Chem. 1996, 61, 4589. (b) Facchetti, A.; Streitwieser, A. J. Org. Chem. 1999, 64, 2281. (41) Eblinger, F.; Schneider, H.-J. J. Phys. Chem. 1996, 100, 5533. (42) Nguyen, H. L.; Horton, P. N.; Hursthouse, M. B.; Legon, A. C.; Bruce, D. W. J. Am. Chem. Soc. 2004, 126, 16. (43) (a) Forni, A.; Metrangolo, P.; Pilati, T.; Resnati, G. Cryst. Growth Des. 2004, 4, 291-295. (b) De Santis, A.; Forni, A.; Liantonio, R.; Metrangolo, P.; Pilati, T.; Resnati, G. Chem.-Eur. J. 2003, 9, 3974. (44) (a) X ) Y ) F, Cl, Br: Chen, G. S.; Anthamatten, M.; Barnes, C. L.; Glaser, R. J. Org. Chem. 1994, 59, 4336. (b) X ) Y ) I: Lewis, M.; Barnes, C.; Glaser, R. J. Chem. Crystallogr. 1999, 29, 1043. (45) Bondi, A. J. Phys. Chem. 1964, 68, 441.

13

C NMR Study of Halogen Bonding of Haloarenes

Scheme 2. Iodoarene-DMSO Halogen Bonding Geometry

around halogen atoms. It was found that nucleophiles in general tend to approach the C-X bond in a “head-on” fashion with Γ ≈ 158(13)° (Cl), Γ ≈ 162(12)° (Br), and Γ ≈ 165(8)° (I). Similar results were obtained by Allen et al.47 The DMSO complexes with the iodoarenes are “head-on” with ∠(O‚‚‚I-C) angles Γ close to 180° as in the case of the iodoalkyne-DMSO complexes (Scheme 2).37 This near-linear head-on coordination contrasts with the Γ angles of 110-140° between DMSO and F3C-I reported by Romaniello and Lelj48 based on DFT studies. Iodine-DMSO bonding thus shows significant intrinsic iododonor specificity, and the variations documented by Parthasarthy et al.46 are not all due to crystal packing. The ∠(S-O‚‚‚I) angles Ω are in the range 125-135°, and there is general agreement that the OdS bond is coordinated in a side-on fashion so that the DMSO oxygen can orient an n-electron density to iodine. The orientation of the S-O bond relative to the benzene plane also is of interest, and we use the ∠(S-O‚‚‚Cipso-Cortho) dihedral angle Λ for its description. The Λ angle falls in the range 55-75° for the complexes of 9-11 and the C2 complexes of 4. A theoretical level dependency occurs for the structure of 12‚DMSO in that it is chiral with Λ ≈ 50° at B3LYP while it has Cs symmetry at MP2 (Λ ) 90°). The same kind of level dependency is seen in the Ci complexes 4‚(DMSO)2. The halogen bond is easily distorted; Figure 3 of the online version of the paper provides links to animations of the six lowest frequency vibrations of the DMSO complex of iodobenzene 12 as Web-enhanced objects. Halogen Binding Energies. We computed binding energies ∆E, enthalpies ∆H, and Gibbs free enthalpies ∆G at several levels, and they are summarized in Table S4 of the Supporting Information. The negative ∆G values indicate that complex formation in the gas phase will not occur because of entropy. In condensed phase, the entropy term is much smaller, and our discussion focuses on binding energies and enthalpies. The binding energies show some theoretical level effects in that MP2 theory shows larger binding than B3LYP theory, and basis set effects are significantly higher at the MP2 level. The halogen bonding is about the same for each halogen bond in the complexes of 4 and 9-12, but the numbers computed at the DFT and MP2 levels still differ by a factor of 2. For iodobenzene 12 we also computed binding energies at levels CCSD(T)/ B//MP2/A and CCSD(T)/C//MP2/A (last row of Table S4). The CCSD(T)/B//MP2/A data are slightly lower than the MP2/B data, and the CCSD(T)/C//MP2/A data give a still lower binding energy that is similar to the DFT data. Our best levels suggest a binding energy of about 3 kcal/mol per iodide-DMSO contact. 13C NMR Chemical Shift Calculations. The results of nonrelativistic GIAO NMR calculations for 4 and 9-12 and for their DMSO complexes are given in the Supporting (46) Ramasubbu, N.; Parthasarathy, R.; Murray-Rust, P. J. Am. Chem. Soc. 1986, 108, 4308. (47) Lommerse, J. P. M.; Stone, A. J.; Taylor, R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108. (48) Romaniello, P.; Lelj, F. J. Phys. Chem. A 2002, 106, 9114.

ARTICLES Scheme 3. Iodine Spin-Orbit Coupling Causes Electron Spin Polarization and Affects the Attached C-Atom by a Fermi Contact Mechanism

Information, and the most pertinent results are summarized in Table 2. Computations were carried out for all systems at the level B3LYP/B//B3LYP/A. For 9-12 and their complexes, calculations also were performed at the MP2(full)/B//MP2/A level, while this proved too demanding for the DMSO complexes of 4. Generally, good agreement is achieved between the measurements and the chemical shift computations with both the MP2 and B3LYP methods. However, neither method reproduces the experimental chemical shift of Ci attached to iodine; these were measured to be δ(Ci) ) 92-102 ppm in DMSO, while the calculations suggested much higher values of 120-143 ppm (Table 2). Moreover, the comparison of the nonrelativistically calculated chemical shifts in the free molecules and their complexes shows noticeable chemical shift changes for Ci (column 5 in Table 2), while the measurements show hardly any change (last column in Table 2). These systematic deviations may be attributed to the neglect of the spin-orbit-induced heavy atom effect of iodine28 in the nonrelativistic GIAO calculations. The so-called “normal halogen dependence” (NHD), that is, the successively larger chemical-shift decrease of light atoms bonded to a heavy element (e.g., halogen) with increasing atomic number of the substituent was described in the 1950s, and spin-orbit coupling was first suggested as its cause in 1969.49 It is now known28,50 that the spin polarization induced by SO coupling at the heavy atom center contributes to NMR shielding tensors predominantly by a Fermi contact mechanism (Scheme 3). Using the same FPT/SOS approach as in the present work29 to compute SO corrections to chemical shifts, Kaupp et al. found an SO-induced heavy atom shielding by -33.3 ppm for Ci in iodobenzene. SO shifts of comparable magnitude are expected for the Ci atoms in the iodo-substituted systems studied here, and it was of interest to investigate how the SO shifts are affected by complexation. The SO corrections calculated here for the Ci atoms (Table 2) are indeed of similar magnitude as for iodobenzene (the effect is known to increase with the s-character of the Ci-I bond).28 They reduce the computed Ci shifts from ca. 130-140 ppm to ca. 90-100 ppm and thus bring them into much better agreement with experiment. Moreover, the SO effects are notably smaller for the free molecules than for the DMSO complexes, and they thus affect the coordination shifts. The nonrelativistic shifts increase by (49) Nomura, Y.; Takeuchi, Y.; Nakagawa, N. Tetrahedron Lett. 1969, 8, 639. (50) For a recent review, see: Kaupp, M. Relativistic Effects on NMR Chemical Shifts. In RelatiVistic Quantum Chemistry; Schwerdtfeger, P., Ed.; Elsevier: Amsterdam, 2003; Vol. 23, in press. J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004 4417

ARTICLES

Glaser et al.

Table 2. Computed and Measured Ci Chemical Shifts and Chemical Shift Changes (in ppm) upon Complexation (Coordination Shifts), with and without SO Corrections system

δ(NR)a

SO correctionsb

δ(NR+SO)

9 9‚DMSO 10 10‚DMSO 11 11‚DMSO 12 12‚DMSO 4 4‚(DMSO)2c

135.6 (124.2) 143.7 (130.9) 127.5 (121.4) 135.6 (128.2) 140.0 (126.3) 148.8 (133.5) 131.6 (123.2) 139.2 (129.5) 134.5 142.5

-34.3 (-36.5) -42.4 (-43.2) -35.2 (-38.8) -43.6 (-44.8) -34.6 (-36.6) -37.7 (-39.0) -31.1 (-36.5) -42.1 (-42.6) -27.8 -33.4

101.3 (88.0) 101.3 (87.7) 92.3 (82.6) 92.0 (83.4) 105.4 (89.7) 111.1 (94.5) 100.5 (86.7) 97.1 (86.9) 106.7 109.1

coordination shift (NR)a

coordination shift (NR+SO)

+8.1 (+6.7)

0.0 (-0.3)

+8.1 (+6.8)

-0.3 (+0.8)

-1.1

+8.8 (+6.8)

+5.7 (+4.8)

0.6

+7.6 (+6.3)

-3.4 (+0.2)

0.4

+8.0

+2.4

0.7

a Nonrelativistic (NR) shifts and coordination shifts at B3LYP/B//B3LYP/A (MP2(full)/B//MP2/A) level from Table S5. level, using B3LYP/A (MP2/A) structure data, respectively. c Based on the Ci symmetric structure of 4‚(DMSO)2.

Table 3. Natural Population Analysis for Hydrazone 10, Acetophenone 11, Iodobenzene 12, and Iodoacetylene at MP2(full)/B// MP2/Aa compd

DMSO 10 10‚DMSO 11 11‚DMSO 12 12‚DMSO H-CtC-I H-CtC-I‚DMSO

q(R−I)

q(Ci)

-0.022 -0.025 -0.022 -0.036

-0.217 -0.246 -0.214 -0.243 -0.215 -0.244 -0.345 -0.354

q(I)

0.168 0.219 0.183 0.235 0.166 0.216 0.314 0.334

q(O)

q(S)

-0.947

1.077

-0.968

1.098

-0.962

1.092

-0.958

1.089

-0.966

1.097

a Basis set A is LANL2DZ. Basis set B is the all-electron basis described in the text.

about 6-8 ppm upon complexation by DMSO (Table 2). In most cases, these positive coordination shifts are offset by the change in the SO contributions, which are more negative for the DMSO complexes than for the free molecules. The two effects compensate each other, and halogen bonding thus has no apparent effect on Ci in the iodoarenes (for 11, the calculations provide somewhat smaller SO effects, and the cancellation is thus incomplete in these data). In the halogen bonding of the iodoalkynes, the two factors are not balanced, and halogen bonding becomes manifest in the NMR chemical shifts.37b Electronic Mechanism of Halogen Bonding of Iodoarenes. Iodoalkynes should be better halogen bond acceptors than iodoarenes because of the higher electronegativity of sp-C compared to sp2-C. Stabilization energies and interaction distances show that 1-iodo-phenylacetylene is more strongly halogen bonded than 10, and this is true for the parent iodoacetylene as well (Table S4). Further corroboration and insight into the halogen bonding mechanism are provided by the results of population analysis (Table 3). The overall charge transfer from the O-donor to the iodide acceptor is very small, less than 0.04, and the charge transfer is almost twice as large for iodoacetylene than for any of the iodoarenes. Aside from a dispersion contribution, the halogen bonding between iodoarenes and DMSO largely is a dipoledipole interaction and synergistically reinforced by the Cδ--Iδ+ and Oδ--Sδ+ bond polarity increases upon halogen bonding (Scheme 4). For the iodobenzenes, we find that the overall charge on R-I is just about equal to the sum of the charge changes on the Ci and I atoms, and halogen bonding with iodoarenes thus is mostly a local event of the Cδ--Iδ+ bond, with only a slightly higher 4418 J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004

coordination shift (exptl)

b

SO corrections added at P86

Scheme 4. Halogen Bonding Mechanism for Iodoarenes

change in |q(Ci)| than in |q(I)|. On the other hand, halogen bonding is a nonlocal event in the iodoacetylene. In this case, the Cδ--Iδ+ polarization is stronger to begin with but the synergistic effect is less pronounced. In particular, there is much less buildup of negative charge at Ci upon halogen bonding. The nonrelativistic GIAO calculations showed an increase in the chemical shift of Ci upon complexation. The charge analysis reveals that this chemical shift increase occurs despite the increased negative charge on carbon. Only the SO contributions decrease the chemical shift of Ci. Conclusion

Solvent effects on the 13C NMR spectra of para-disubstituted acetophenone azines X-C6H4-CMedN-NdCMe-C6H4-Y are generally small. Independent of the choice of the solvent, hardly any electronic communication is found between the two halves of the unsymmetrical azines, and the spectroscopic properties of the symmetrical azines carry over to the unsymmetrical azines. Our previous 1H and 13C NMR spectroscopic study of azines in chloroform suggested that the azine bridge functions as a “conjugation stopper”, and the results presented here provide important corroboration. Hence, one of our important assumptions for the design of polar materials, the assumption that the azine bridge functions as a “conjugation stopper”, is now firmly established by both theory and experimentation. In the absence of evidence for solute-solvent interactions in the NMR spectra of the haloarenes, the question occurs as to whether there are no specific interactions or, if specific solvation occurs, why there is no manifestation of the specific solvation in the chemical shifts. The complexation of iodoarenes by DMSO was studied as iodine bonding should be one of the stronger intermolecular interactions and because of the prior evidence that DMSO complexation does cause NMR shifts for some iodides. DFT and MP2 theory show the same characteristics of the iodine bonding between iodoarenes and DMSO, and similar structures are obtained with the following features: (1) The O‚‚‚I

13

C NMR Study of Halogen Bonding of Haloarenes

distances in all complexes are close to 2.9 Å and about 0.6 Å shorter than the sum of the van der Waals radii. (2) The DMSO-oxygen approaches the C-I bond in a “head-on” fashion with ∠(O‚‚‚I-C) angles Γ close to 180°. (3) The ∠(S-O‚‚‚I) angles Ω are in the range 125-135°, and there is general agreement that the OdS bond is coordinated in a sideon fashion so that the DMSO oxygen can orient n-electron density toward iodine. The binding energies show some dependencies on the method and the basis set, and there clearly is a need for measurements of binding energies. Our best levels suggest a binding energy of about 3 kcal/mol per iodide-DMSO contact. The population analysis shows the halogen bonding between iodobenzene and DMSO to be a local event. IodineDMSO bonding largely is a dipole-dipole interaction which is synergistically reinforced by C δ--Iδ+ and Oδ--Sδ+ bond polarity increases upon halogen bonding. Good agreement is generally achieved between the measurements and the nonrelativistic GIAO NMR chemical shift computations with both the MP2 and B3LYP methods. However, neither method reproduces the experimental chemical shift of Ci attached to iodine because of the neglect of the spinorbit-induced heavy atom effect of iodine in the nonrelativistic GIAO calculations. We computed the SO corrections for the Ci atoms, and these corrections bring the computed Ci shifts in much better agreement with experiment. The nonrelativistic shifts increase by about 6-8 ppm upon complexation by DMSO. In most cases, these positive coordination shifts are offset by the change in the SO contributions, which are more negative for the DMSO complexes than for the free molecules. The two

ARTICLES

effects compensate each other, and halogen bonding thus has no apparent effect on Ci in the iodoarenes. For the iodides, complexation by DMSO occurs and may or may not manifest itself in the NMR spectra. Halogen bonding by other halides is weaker and less likely in solution; there are no reports of complexation effects on their NMR spectra, and we have not observed any such effects in the present study. In any case, the absence of complexation shifts in the NMR spectra does not exclude the occurrence of halogen bonding. Acknowledgment. This work was supported by the UM Research Board, the MU Research Council, and the United States National Institutes of Health (GM61027). We thank Drs. Gordon Springer and Larry Sanders for their research computer systems support. Work in Wu¨rzburg was supported by the Deutsche Forschungsgemeinschaft (KA 1187/5-1), by the Fonds der chemischen Industrie, and by the Graduiertenkolleg 448 “Moderne Methoden der magnetischen Resonanz in der Materialforschung” of the Universita¨t Stuttgart. Supporting Information Available: Four figures showing the HMQC, 13C NMR, APT, and the DEPT spectra of 4 in DMSO, two tables with measured NMR data of the symmetrical azines and the model compounds, four figures showing models of optimized structures (B3LYP and MP2, isolated and complexed), two tables providing the total energies and thermochemical data and binding energies, respectively, and one table of computed NMR shifts (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. JA0383672

J. AM. CHEM. SOC.

9

VOL. 126, NO. 13, 2004 4419